Background Understanding and Suppression in Very Long Baseline Neutrino Oscillation Experiments with Water Cherenkov De

1. Background Understanding andSuppression in Very LongBaseline Neutrino Oscillation Experiments with Water Cherenkov Detector Chiaki Yanagisawa Stony Brook Talk at NNN05, Aussois, France April 7-9, 2005

2. Very long baseline neutrino oscillation VLBNO Setting the stage ~ a half megaton F.V. water Cherenkov detector, for example UNO BNL very long baseline neutrino beam VLB neutrino oscillation experiment See, for example, PRD68 (2003) 12002 for physics argument How do we find the signal for nve nm ne and ne +N e + invisible N' + (invisible n s, n0) Look for single electron events g (g) Major background nm,t,e + N nm,t,e + N' + p0 + (invisible n s, n0) necontamination in beam (typically 0.7%)

3. VLBNO Neutrino spectra of on- and off-axis BNL Superbeams PRD68 (2003) 12002; private communication w/ M.Diwan on-axis beam 1 o off-axis beam nms/GeV/m2/POT Neutrino energy (GeV)

4. VLBNO How is analysis done ? Use of SK atmospheric neutrino MC special p0 finder • Standard SK analysis package + • Flatten SK atm. n spectra and reweight with BNL beam spectra • Normalize with QE events: 12,000 events for nm , 84 events for beam ne for 0.5 Mt F.V. with 5 years of running, 2,540 km baseline distance from BNL to Homestake • Reweight with oscillation probabilities for nmand for ne Oscillation parameters used: • Dm221 =7.3 x 10- 5 eV2, Dm231=2.5 x 10- 3eV2 • sin22qij(12,23,13)=0.86/1.0/0.04, dCP=0,+45,+135,-45,-135o Probability tables from Brett Viren of BNL

5. VLBNO Selection criteria used to improve Traditional SK cuts only Initial cuts: • One and only one electron-like ring with energy and reconstructed neutrino energy more than 100 MeV without any decay electron To reduce events with invisible charged pions With p0 finder Likelihood analysis using the following eight variables: • p0mass, energy fraction, costh, p0-likelihood, e-likelihood • Dp0-likelihood, total charge/electron energy, Cherenkov angle

6. VLBNO What are sources of the signal (QE and nonQE) ? Only single e-like events left after initial cut Reconstructed energy Reconstructed energy QE events only before likelihood cut All CC events before likelihood cut En En Erec Erec Erec Erec All CC events that survive the initial cuts are signals

7. p0Finder p0 finder Always looks for an extra ring in a single ring event p0finder p0reconstruction efficiency with standard SK software measured opening angle vs. p0mass with p0finder inefficiency due to overlap inefficiency due to weak 2nd ring Single e-like events from single p0int. All single p0interactions SK atm. neutrino spectra opening angle measured(deg) efficiency mgg (MeV/c2) true opening angle (deg)

8. p0finder p0reconstruction efficiency p0reconstruction efficiency with standard SK + p0finder All the single p0int. with p0 finder w/o p0 finder with p0 finder p0 mass cut:1- and 2-ring events With atmospheric neutrino spectra efficiency without p0 finder p0 mass cut:2-ring events True opening angle (deg)

9. All the distributions of useful variables are obtained with neutrino oscillation “on” with CPV phase angle +450 • Useful Variables Variables p0 mass 0.0-0.5 GeV 0.5-1.0 GeV 2.0-2.5 GeV 2.5-3.0 GeV background signal p0 mass p0 mass p0 mass p0 mass 1.0-1.5 GeV 1.5-2.0 GeV 3.0-3.5 GeV 3.5-4.0 GeV p0 mass p0 mass p0 mass p0 mass

10. Variables Fake ring has less energy than real one Energy fraction of 2nd ring 0.0-0.5 GeV 0.5-1.0 GeV background 2.0-2.5 GeV 2.5-3.0 GeV signal 1.0-1.5 GeV 1.5-2.0 GeV 3.0-3.5 GeV 3.5-4.0 GeV

11. Variables Difference between ln of two p0-likelihood (wide vs. forward) - One algorithm optimized to find an extra ring near the primary ring (forward region) - Another algorithm optimized to find an extra ring in wider space (wide region) - See the difference ln p0-likelihood (forward) - ln p0-likelihood (wide) Primary electron ring An undetected weak ring initially

12. Variables Difference between ln of two p0-likelihood (wide vs. forward) 0.0-0.5 GeV 0.5-1.0 GeV 2.0-2.5 GeV 2.5-3.0 GeV background signal 1.0-1.5 GeV 1.5-2.0 GeV 3.0-3.5 GeV 3.5-4.0 GeV

13. Variables costh = cos qe 0.0-0.5 GeV 0.5-1.0 GeV 2.0-2.5 GeV 2.5-3.0 GeV background signal 1.0-1.5 GeV 1.5-2.0 GeV 3.0-3.5 GeV 3.5-4.0 GeV

14. Trained with ne CC events for signal, nm CC/NC & ne,t NC for bkg Likelihood Cut Difference in ln likelihood between sig and bkg D ln likelihood distributions ln likelihood ratio 0.0-0.5 GeV 0.5-1.0 GeV 2.0-3.0 GeV 3.0- GeV signal background Preliminary Preliminary D likelihood D likelihood D likelihood D likelihood 1.0-1.5 GeV 1.5-2.0 GeV D likelihood D likelihood

15. Signal/Background • ne CC for signal ; all nm,t,e NC , ne beam • for background Effect of cut on D ln likelihood After initial cuts No D ln likelihood cut (100%signal retained) D ln likelihood cut (~50%signal retained) TRADITIONAL ANALYSIS Preliminary Preliminary Background from p0 o CP+45 o CP+45 ne background Signal Erec Erec Signal 700 ev Bkgs 2005 (1878 from p 0+others) ( 127 from ne) Signal 321 ev Bkgs 169 (112 from p 0+others) ( 57 from ne)

16. Signal/Background • ne CC for signal ; all nm,t,e NC , ne beam • for backgrounds Effect of cut on D likelihood D ln likelihood cut (40%signal retained) D ln likelihood cut (~40%signal retained) Preliminary Preliminary Background from p0 o o CP+45 CP-45 ne background Signal Erec Erec Signal 251 ev Bkgs 118 ( 74 from p 0+others) ( 44 from ne) Signal 142 ev Bkgs 118 ( 75 from p 0+others) ( 43 from ne)

17. Signal/Background • ne CC for signal ; all nm,t,e NC , ne beam • for backgrounds Effect of cut on likelihood D ln likelihood cut (~40%signal retained) D ln likelihood cut (~40%signal retained) Preliminary Preliminary Background from p0 o o CP+135 CP-135 ne background Signal Erec Erec Signal 342 ev Bkgs 126 ( 81 from p 0+others) ( 45 from ne) Signal 233 ev Bkgs 122 ( 78 from p 0+others) ( 44 from ne)

18. Issues Some issues Summary of BNL superbeam@UNO Neutrino oscillation was on to define template distributions For analysis CPV=+45o • S/B and variables Variable removed Bkg Beam ne Signal Bkg Effic Signal ne CC nm all, ne,nt NC 50% 2.86 321 112 57 None Dpi0lh 119 nm all, ne,nt NC 1.80 ne CC 321 59 50% poa 2.51 nm all, ne,nt NC ne CC 50% 316 126 56 ne CC 50% 303 116 52 nm all, ne,nt NC 2.61 pi0-lh Preliminary 311 e-lh 2.53 ne CC 50% 127 55 nm all, ne,nt NC 50% ne CC 333 167 60 nm all, ne,nt NC efrac 1.99 pi0mass ne CC 50% 310 143 56 nm all, ne,nt NC 2.17 57 146 nm all, ne,nt NC costh 322 2.21 ne CC 50% ange 321 119 55 2.70 ne CC nm all, ne,nt NC 50%

19. Conclusion Conclusion Realistic MC simulation studies have been performed for the BNL very long baseline scenario with a water Cherenkov detector. It was found that BNL VLB combined with a UNO type detector seems to DO A GREAT JOB – Very exciting news but needs confirmation. It was demonstrated that there is some room to improve S/B ratio beyond the standard water Cherenkov detector software even with currently available software We may need further improvement of algorithm/software, which is quite possible Detailed studies on sensitivity on oscillation parameters needed A larger detector such as UNO has an advantage over a smaller detector such as SK (we learned a lesson from 1kt at K2K)